Grade 403 is a martensitic stainless steel built around roughly 12% chromium with low carbon content, placing it in the same chromium-only family as 410 but with tighter compositional control aimed at forging and turbine-quality applications. Like other martensitic grades, 403 has no nickel to speak of and gets its strength from heat treatment rather than cold work, which sets it apart from the austenitic 300-series stainless steels. Its lower carbon content compared with 410 or 420 gives it noticeably better toughness and ductility at a given hardness, at the cost of the higher hardness ceiling those grades can reach.

The combination of moderate strength, good toughness, and reasonable corrosion resistance makes 403 a common choice for steam and gas turbine blading, compressor components, valve parts, and fasteners that need to hold up in mildly corrosive service while resisting fatigue and impact. It machines somewhat more easily than higher-carbon martensitic grades — published machinability ratings put it around 55% relative to a free-machining reference steel — but its ductility means chip control, rather than raw hardness, is usually the main challenge at the machine.

403's low carbon content compared with other martensitic stainless grades makes it noticeably more ductile and tougher, which is a double-edged sword at the machine. On one hand, it doesn't have the abrasive, tool-dulling hardness of a high-carbon martensitic grade like 420. On the other, that same ductility means the material shears cleanly but tends to form long, stringy, hard-to-break chips rather than the short segmented chips a harder alloy would produce. Chip control, not tool wear, is usually the operator's biggest headache with this grade.

Because 403 is chromium-only with no significant nickel, it doesn't work-harden as aggressively as the austenitic 300-series grades, but it still benefits from a sharp cutting edge that shears the material instead of pushing and rubbing it. A tight, well-matched chip breaker geometry is essential for keeping long ribbons of chip from tangling around the tool or workpiece. Many insert grades developed for austenitic stainless steels such as 304 and 316 also perform well on 403, since both material families reward a sharp edge and clean shearing action even though their hardening behavior differs.

At stable cutting conditions, a semi-hard substrate with a thin CVD coating is a solid general-purpose choice for turning. If cutting speeds drop below roughly 390 SFM (120 m/min), a semi-hard substrate with a PVD coating tends to hold up better. For milling, a semi-hard substrate with a thin PVD coating is generally preferred.